Hi,
I assume you are also connecting power to the second board , not just the data signal connections ?
Is the USB cable from a desktop pc or laptop ? - there is a limit as to how much power a usb cable can supply.
I am not familiar with those boards, but if you have a test meter you need to measure the voltage and current to each board when connected separately, then with both boards connected together.
It sounds like you either have a short circuit on one board or the total power needs exceed the usb cable.